Age and diseases such as osteoporosis mean that many older people can be frail and have a much slower walking pace than they did in their youth.

Fear of not being able to cross main roads in time could leave many frightened of leaving their homes or of walking anywhere except short distances, researchers warn.

A new study, which will be presented at a conference organised by the British Geriatrics Society in Edinburgh next week, found that half of people over the age of 80 struggled to make it across pedestrian crossings in time.

It asked 355 people of varying ages over 60 to take part in a series of laboratory experiments to test how fast they walked.

Based on the results around half of over 80s would not be able across pedestrian crossings in time.

Many of them were able to walk only at speeds less than 1 metre a second, far below the average 1.2 metres a second walking speed used to calculate pedestrian crossing timings.

Prof Rose Anne Kenny, from Trinity College, Dublin, who led the project, said: “Overall, we estimate that this is a problem for 50 per cent of those aged over 80.

“With increasingly ageing populations more and more people are going to struggle.

“The group we studied were actually slightly healthier than the average for their age.

“And we did not take into account other people who may be less able to cross the road quickly, including pregnant women and those with disabilities.”

She added: “Adding an extra second or two would make a big difference.

“We suggest that longer crossing times could be used during the day, say between 11am and 3pm, when older people may want to make journeys but also the change would have less impact on traffic.”

Pedestrian crossings are made up of two parts.

The first, in which a steady green man is shown, is shown is known as an 'invitation to cross'.

This period, which indicates that it is safe for a pedestrian to start walking but it not intended to be long enough for them to make it to the other side, should last between three and 15 seconds.

The second part, when either a flashing green man is shown or the screen goes dark, should be long enough to allow a pedestrian who steps off the kerb as the green man changes to finish crossing safely.

Depending on the size of the road and other factors, including the average number of pedestrians, this part should last between three and 15 seconds.

The average walking speed used to calculate timings should be 1.2 metres per second, according to the Department for Transport.

Motoring experts warned that lengthening crossing times could have unintended consequences and lead to roads becoming more dangerous.

Andrew Howard, head of road safety at the AA, said: “It is a difficult balancing act.

“If drivers are held at the lights for a long time and they see a healthy, 50-something scramble across the road, they might start to think that they are being stopped for no reason and go anyway.”

Figures show that 187 pedestrians over the age of 70 died on the roads in 2008.

During the same year 149 drivers over 70 were also killed.

Earlier this week the RAC Foundation called for road signs to carry larger lettering to help Britain's ageing motorists.